Bug#551786: linux-image-2.6.26-2-686-bigmem: disk write performance regression

[Holger Weiss <holger@zedat.fu-berlin.de>]

Package: linux-image-2.6.26-2-686-bigmem
Version: 2.6.26-19
Severity: normal
File: /boot/vmlinuz-2.6.26-2-686-bigmem
Tags: patch

We encountered a serious disk write performance regression after
upgrading from the etch kernel (2.6.18) to the etchnhalf kernel
(2.6.24).  Co-workers of mine stumbled over the issue using OpenLDAP
(with the Berkeley DB backend) while restoring about 60,000 medium-sized
records with slapadd.  As soon as they define an "index" with "sub" in
their slapd.conf, the restore performance goes south and the load
increases dramatically.  The restore won't be finished after half an
hour, while it's done within a few minutes when runnning 2.6.18.  We
were able to reproduce the problem on systems running lenny (with the
stock 2.6.26 kernel), on different hardware, with different file
systems, and with different OpenLDAP versions (we tested this with
OpenLDAP 2.3.30, 2.4.13, 2.4.16, 2.4.18, and 2.4.19).  However, I'm not
aware of another way to trigger the issue, sorry.

The problem is not Debian-specific.  It was introduced with the kernel
Git commit v2.6.21-1-g07db59b, which lowered the default dirty-writeback
limits; and has been fixed with commit v2.6.29-4-g1b5e62b, which raised
these settings again (though not to the original values).  Backporting
the new settings from v2.6.29-4-g1b5e62b fixes the problem for us.  The
attached patch should be applicable to both 2.6.26 from lenny and 2.6.24
from etchnhalf.

--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -66,7 +66,7 @@ static inline long sync_writeback_pages(void)
 /*
  * Start background writeback (via pdflush) at this percentage
  */
-int dirty_background_ratio = 5;
+int dirty_background_ratio = 10;
 
 /*
  * free highmem will not be subtracted from the total free memory
@@ -77,7 +77,7 @@ int vm_highmem_is_dirtyable;
 /*
  * The generator of dirty data starts writeback at this percentage
  */
-int vm_dirty_ratio = 10;
+int vm_dirty_ratio = 20;
 
 /*
  * The interval between `kupdate'-style writebacks, in jiffies